1. Field of the Invention
The present invention relates to a semiconductor laser element and a semiconductor laser, and more specifically to a semiconductor laser element and a semiconductor laser having an improved heat dissipation characteristic. The present invention also relates to a semiconductor laser element comprising a GaN-base semiconductor and to a semiconductor laser provided with such a semiconductor laser element.
2. Description of the Prior Art
Recently, the use of semiconductor lasers has been expanding remarkably In many of these uses, there has been a demand for a semiconductor laser with higher output. Accordingly, for realizing higher output of a semiconductor laser, various attempts have been made to improve the structure of the semiconductor laser element itself. For example, reported in Literature 1), J. K. Wada et al, xe2x80x9c6.1 W continuous wave front-facet power from Al-free active-region (xcex=805 nm) diode lasersxe2x80x9d, Applied Physics Letters, Vol. 72, No. 1 (1998) pp. 4-6, is a semiconductor laser having an active layer made of InGaAsP containing no Al, an optical waveguide layer made of Ingap, and a clad layer made of InAlGap, and oscillating in a 805 nm band.
Suggested in Literature 1) as the structure for lowering the photodensity of the active layer to improve the high output characteristic is an LOC (Large Optical Cavity) structure with widened thickness of the optical waveguide layer. The increase of the maximum light output thereby is reported.
Also known as the semiconductor laser having an active layer with no Al and oscillating in a 0.8 xcexcm band is a semiconductor laser having, on an n-GaAs substrate, an n-AlGaAs clad layer, an i-InGaP optical waveguide layer, an InGaAsP quantum well active layer, an i-InGaP optical waveguide layer, a p-AlGaAs clad layer, and a p-GaAs cap layer, as shown in Literature 2); T Fukunaga et al. xe2x80x9cHighly Reliable Operation of High-Power InGaAsP/InGaP/AlGaAs 0.8 xcexcm Separate Confinement Hetrostructure Lasersxe2x80x9d, Jpn. J. Appl. Phys. Vol. 34, (1995) pp. L1175-1177.
Also, to improve the heat dissipation effect of a semiconductor laser element, various structures of forcibly cooling the device with a cooling medium such as water have already been proposed. For example, proposed in Literature 3), Ray Beach et al., xe2x80x9cModular Michrochannel Cooled Heatsinks for High Average power Laser Diodexe2x80x9d, IEEE JOURNAL OF QUANTUM ELECTRONICS, Vol. 28. No. 4, April 1992, is a structure of a water-cooling mechanism for a semiconductor laser element using a microchannel.
On the other hand, about a semiconductor laser of a 400 nm band having a fine spot, required in the field of printing, etc., using an optical disk memory and a photosensitive material, is a beam having a high reliability and a high quality which oscillates in the optical density fundamental transverse mode with a Gauss distribution so as to fit for the increase of the image density and the increase of the image quality. For example, reported in Nakamura et al., Literature 4) InGaN/GaN/AlGaN-Based Laser Diodes Grown on GaN Substrates with a Fundamental Transverse Mode, described in xe2x80x9cJpn. J. Appl. Phys. Lett., Vol. 37, pp. L1020, is a semiconductor laser obtained by depositing an n-GaN buffer layer, an n-InGaN clack-preventing layer, an n-AlGaN/GaN-modulated dope supper lattice clad layer, an n-GaN optical waveguide layer, an m-InGaN/InGaN multiple quantum well active layer, a p-AlGaN carrier block layer, a p-AlGaN/GaN modulated dope super lattice clad layer, and a p-GaN contact layer on a GaN substrate, the Gan substrate being obtained by forming a GaN layer on a sapphire substrate utilizing the selective growth using SiO2 as a mask and by releasing the GaN layer and a part of the sapphire substrate as the Gan substrate.
However, the structure shown in Literature 1) may lead to a phenomenon of COMD (Catastrophic optical mirror damage) , where the end face of the device is damaged because of temperature increase of the end face triggered by an electric current generated by the light absorption at the end face, and because of smaller bandgap which increases light absorption at the end face. Therefore, the maximum light output is restrained to avoid the COMD. Since the light output which may trigger the COMD changes time-by-time, it sometimes happens that the operation of the semiconductor laser is suddenly stopped. As a result of such circumstances, it is difficult to obtain a high reliability at the time of a high output driving in the semiconductor laser proposed by Literature 1).
Also, in the semiconductor laser shown in Literature 2), the maximum light output is considerably low, in fact as low 1.8 W.
Furthermore, in the structure described in Literature 3) above, there is a problem that a large-scale cooling mechanism is required even in the case of cooling a single semiconductor device. Such a large scale cooling mechanism also requires a large place. Another problem is that it is difficult to obtain the sufficient cooling effect required for the recent high-output laser element because cooling is indirectly carried out from one surface connected to a module.
On the other hand, in the semiconductor device described in Literature 4), the reduction of the element resistance is attempted using a modulation dope super lattice clad layer but the reduction is not insufficient. Thus, the deterioration of the reliability due to the joule heat occurs during operation. Also, the resistance of the element is high in the semiconductor laser comprised of the semiconductor layers of the above-described system. Therefore, particularly in the laser of a single mode where the contact area with the contact layer is narrow, the influence of heat generation becomes a problem in practical use. The generation of the joule heat is coped with by cooling using a heatsink, etc. However, in the structure of the above-described semiconductor device, the heat dissipation is insufficient, because a heatsink is formed on the n-GaN layer exposed by etching the side surface of the semiconductor laser device making the form of the device complicated. That is, cooling from the p-electrode side near the active layer generating heat is difficult, and only cooling from the n-electrode side far from the active layer is possible. Also, because the p-electrode and the n-electrode are not disposed in a vertical direction but are disposed to the right and left, the stream of the electric current injected from the p-electrode is not straight but is liable to become inhomogeneous, whereby a uniform light emission where the optical density is a Gauss-type distribution cannot be obtained. For obtaining the light emission of a Gauss-type distribution, it is necessary to make the ridge width as narrow as possible. However, when the ridge width is narrowed, there is a problem that increase of the output is difficult.
The present invention has been made in view of the above-described circumstances and an object of the invention is to provide a semiconductor laser element having a high reliability even under high output.
Also, in view of the above-described circumstances, another object of the invention is to provide a semiconductor laser having a high reliability even under high output by improving the heat dissipation of a semiconductor laser element.
Furthermore, in view of the above-described circumstances, still another object of the invention is to provide a semiconductor laser element and a semiconductor laser, which realize a uniform and high-quality beam having the optical density of a Gauss-type distribution and thus have a high reliability under high output, by improving the heat dissipation and by making the injection of an electric current into the active layer uniform.
That is, the semiconductor laser element according to the first aspect of the invention is a semiconductor laser element having a plurality of semiconductor layers formed on a substrate, comprising
a concaved portion formed on a surface of the substrate, said surface being opposite to the surface having the semiconductor layers formed thereon, wherein
a metal having a higher heat conductivity than the substrate is filled in the concaved portion.
Also, the semiconductor laser element according to the second aspect of the invention is a semiconductor laser element having a plurality of semiconductor layers formed on a substrate, comprising
a concaved portion formed at least in a part of a surface of the semiconductor layer, said surface being opposite to the surface on the side of the substrate, wherein
a metal having a higher heat conductivity than the semiconductor layer is filled in the concaved portion.
In addition, it is desirable that in the semiconductor laser element of the second aspect, as well as the semiconductor laser element of the first aspect, another concaved portion is formed on a surface of the substrate opposite to the surface having the semiconductor layers formed thereon, and that a metal having a higher heat conductivity than the substrate is filled in the concaved portion.
Also, in the semiconductor laser elements of the invention described above, it is desirable that the concaved portion formed the substrate on the semiconductor layer has a reverse mesa form with respect to the direction perpendicular to the light-emitting face.
Also, it is desirable that a heatsink is bonded to the metal filled in the concaved portion.
Furthermore, it is desirable that in the semiconductor laser elements of the invention described above, plural light-emitting portions are formed on the semiconductor layer to constitute a semiconductor laser array.
Moreover, it is desirable that the semiconductor laser elements of the invention described above are used as the light sources for exciting a solid laser.
In the semiconductor laser elements according to the first aspect of the invention described above, where a metal having a higher heat conductivity than the substrate is filled in the concaved portion formed on the substrate, the heat generated at the active layer and the vicinity thereof is dissipated through the metal more efficiently than through the substrate only. Thus, the temperature of the semiconductor laser element (more practically, the temperature in the vicinity of the active layer) is kept low, whereby a high reliability is insured even under a high output operation.
Also in the semiconductor laser element according to the second aspect of the invention, where a metal having a higher heat conductivity than the semiconductor layer is filled in the concaved portion formed on the semiconductor layer, the heat generated at the active layer and the vicinity thereof is dissipated through the metal more efficiently than through the semiconductor layer only. Thus, in the construction, the temperature of the semiconductor laser element (more practically, the temperature in the vicinity of the active layer) is kept low, whereby a high reliability is insured even under a high output operation.
Also, in the case where the semiconductor laser element of the second aspect has the concaved portion which is filled with a metal having a higher heat conductivity than the substrate and which is formed in the substrate as in the case of the semiconductor laser element of the first aspect, the heat dissipating effect from the substrate side is also improved. Thus, the temperature of the semiconductor laser element is kept lower, and still higher reliability is insured under a high output operation.
Also, in the case where the semiconductor laser elements of the invention described above have the concaved portions formed in reverse mesa forms on the substrate and the semiconductor layer, the above-described metal can be arranged up to the portion closer the light-emitting face of the semiconductor laser element. Thus, the light-emitting face, which is liable to become high temperature, can be effectively cooled.
Furthermore, in the semiconductor laser elements of the invention described above having a heatsink bonded to the metal filled in the concaved portion, the heat conducted through the metal can be efficiently dissipated via the heatsink in a three-dimensional manner, whereby a higher reliability can be obtained even under a high output operation.
On the other hand, the semiconductor laser device according to the third aspect of the invention is comprised of
a semiconductor laser element,
a heatsink disposed in contact with the semiconductor laser element,
a cooling medium passageway formed between the heatsink and the semiconductor laser element using at least a part of each of them as a passageway wall, and
means for transferring a cooling medium through the cooling medium passageway.
In addition, the term xe2x80x9ca cooling medium passageway is formed between the heatsink and the semiconductor laser elementxe2x80x9d as described above does not always mean that the cooling medium passageway is formed separately from the heatsink but also includes the state where the cooling medium passageway is formed using a part of the heatsink and the cooling medium passageway covers the area from the rest portion of the heatsink to the semiconductor laser element.
In the above-described construction, it is desirable that the heat sink includes a first heatsink disposed in contact with the substrate of the semiconductor laser element and a second heatsink disposed in contact with the surface of the semiconductor laser element opposite to the substrate side, wherein passageways are respectively formed between each of the first and second heatsinks and the semiconductor laser element first and second cooling medium.
Also, it is preferred that at least a part of the cooling medium passageway is constituted of the groove formed in the substrate of the semiconductor laser element. Also, it is preferred that the groove formed in the substrate is formed in a reverse mesa form. Furthermore, at least a part of the cooling medium passageway may be constituted of a ridge groove formed on the surface of the semiconductor laser element opposite to the substrate.
Also, it is desirable that the wall portion of the groove formed on the semiconductor laser element as described above is coated with a dielectric film such as an SiO2 film, an SiN film, an Al2O3 film, etc.
On the other hand, it is desirable that in the heatsink there are formed a supply passageway for supplying a cooling medium to the above-described cooling medium passageway by connecting thereto and/or a discharge passageway for discharging the cooling medium from the cooling medium passageway.
Also, it is desirable that the heatsink is bonded to the semiconductor laser element using a brazing material having a resistance to the cooling medium.
Furthermore, it is desirable that in the semiconductor laser described above, plural semiconductor laser elements are formed to construct a semiconductor laser array and, on the other hand, the cooling medium passageway connects each of the plural semiconductor laser elements and the heatsink.
Also, it is desirable that the semiconductor laser element(s) constituting the semiconductor laser of the invention are used as the light sources for exciting a solid laser.
Because, in the semiconductor laser of the invention described above, the cooling medium passageway is formed between the heatsink and the semiconductor laser element using at least a part of each of the heatsink and the semiconductor laser element as the wall of the passageway and the apparatus is constructed such that a cooling medium flows through the cooling medium passageway, the cooling medium flows while directly contacting with the semiconductor laser element and the heatsink. Thus, the heat dissipation from the semiconductor laser element to the heatsink is carried out sufficiently well via the cooling medium, the heat-dissipating characteristics of the semiconductor laser element (more practically, the vicinity of the active layer) are improved, and a high reliability is insured even under a high-output operation.
In addition, in the case where the semiconductor laser of the invention described above has the first and second heatsinks formed as described above, and where first and second cooling medium passageways are formed between the semiconductor laser element and the first and second heatsinks respectively, the heat dissipating characteristics are remarkably improved and the reliability under a high output operation is further increased.
Also, in the case where the semiconductor laser of the invention described above has at least a part of the cooling medium passageway constructed of the groove formed on the substrate of the semiconductor laser element, or of the ridge groove formed on the surface of the semiconductor laser element opposite to the substrate, the structured of the cooling medium passageway becomes simple and can be formed in a small size. Therefore, the structure of the semiconductor laser can be simplified and miniaturized.
In addition, when the groove wall portions of the groove formed in the semiconductor laser element are coated with the dielectric, it prevents problems such as short-circuiting, etc., which may be caused by the direct contact of the cooling medium with the portion conducting an electric current of the semiconductor laser element.
Furthermore, in the case where the semiconductor laser of the invention described above has the supply passageway for supplying the cooling medium to the cooling medium passageway by connecting thereto and/or the discharge passageway for discharging the cooling medium from the cooling medium passageway is formed in the heatsink, the structures of the supply passageway and the discharge passageway for the cooling medium can be simplified. Therefore, the structure of the semiconductor laser can be simplified and miniaturized.
In addition, when the heatsink is bonded to the semiconductor laser element using a brazing material having a resistance to the cooling medium, the occurrence of the problem that the brazing material is deteriorated by the cooling medium to weaken the bonding between the heatsink and the semiconductor laser element can be prevented.
Further, in the case where the semiconductor laser of the invention described above has a plurality of semiconductor laser elements constituting a semiconductor laser array and where the cooling medium passageway connects each of the semiconductor laser elements and the heatsink, each of the semiconductor laser elements can be efficiently cooled and the reliability of the semiconductor laser array under high-output operation can be increased.
Also, the semiconductor laser element according to another aspect of the invention is a semiconductor laser element comprising a GaN substrate provided with one of a pair of electrodes, a semiconductor layer which is made of a GaN-base semiconductor containing at least an active layer and which is disposed on the GaN substrate, the other of the pair of electrodes disposed on the semiconductor layer, and an electric current injection region for injecting an electric current onto the semiconductor layer, wherein a groove is formed on a region of the GaN substrate corresponding to the electric current injection region to a depth reaching the semiconductor layer from the surface of the substrate opposite to the side of the semiconductor layer, and wherein one of the electrodes is formed on the surface of the groove.
In this case, it is desirable that a contact layer is formed on the GaN substrate side of the semiconductor layer, and the contact layer is ohmic-connected to one of the electrodes formed on the surface of the groove.
Also, it is desired that the surface of the groove filled with a metal having a higher heat conductivity than GaN is flattened and a heatsink is bonded to the flattened surface. It is preferred that the metal is Au.
Also, the semiconductor laser device according to another aspect of the invention comprises the semiconductor laser element of the present invention according to the above-described construction, a heatsink which is connected to the GaN substrate side of the semiconductor laser element and which has a supply passageway for supplying a cooling medium to a groove and a discharge passageway for discharging the cooling medium from the groove, and means for transferring a cooling medium through the groove via both the passageways.
In addition, the groove may be formed along an entire length from one end face vertical to the resonance direction of light to the other end face, or may be a part between one end face and the other end face.
Also, the above-described one of the electrodes may be formed so as to not only the inside surface of the groove but also the area between the inside of the groove and the surface opposite to the surface laminated with the semiconductor layer of the GaN substrate.
In addition, the above-described GaN-base semiconductor a semiconductor including Ga and N as the constituent features, for example, GaN, InGaN or AlGaN.
According to the semiconductor laser element of the invention described above, where a groove is formed at the region of the GaN substrate corresponding to the electric current injection region to a depth reaching the semiconductor layer and where one of the electrodes is formed on the surface of the groove, the construction thereof includes a pair of electrodes formed at both end faces in of the laminated layer direction making the passage for the electric current straight. Thus, the electric current is uniformly injected onto the active layer, whereby a uniform oscillation mode with the optical density of a Gauss-type distribution can be obtained. Therefore, unlike in the prior art, the output can be increased without narrowing the ridge width for making the optical density uniform. Thus, it is possible to increase the output and to obtain the oscillation beam having a high quality and a high reliability.
In addition, because the side surface of the semiconductor laser element was etched to the n-GaN layer and an n-electrode was formed thereon in the semiconductor laser element using the GaN substrate of prior art, the form of the semiconductor laser element was complicated. Thus, the p-electrode side near the active layer could not connect (p-down bonding) to the heatsink, and cooling was carried out from the n-electrode side. However, according to the semiconductor laser element of the invention, it becomes possible to connect the p-electrode side near the active layer to the heatsink, the cooling effect is increased, and the oscillation mode of a high quality can be obtained even under high output operation. Alternatively, it becomes possible to connect the n-electrode side to the heatsink (n-down bonding). Furthermore, it is possible to connect both the electrodes to the heatsink, whereby the cooling effect is further increased and the oscillation mode of a high quality can be obtained even under high output operation.
Also, as the contact layer is formed on the GaN substrate side of the semiconductor layer and as the contact layer and one of the electrodes formed on the surface of the groove forms an ohmic-connection, the resistance of the semiconductor laser element can be reduced. Thus, the influence of the heat generation is restrained and the beam of a high quality can be obtained.
Also, because the groove is filled with a metal having a higher heat conductivity than GaN, the surface of the side having the groove is flattened, and the heatsink is connected to the flattened surface, the semiconductor laser element is uniformly connected to the heatsink. Therefore, the heat dissipating characteristics are improved to restrain the influence of the heat generation. Also, good heat dissipation can be obtained because the metal filled in the groove is Au.
Furthermore, the heat can be more efficiently dissipated and the oscillation mode of a high quality can be obtained even under high output operation, according to the semiconductor laser of the invention described above comprising the semiconductor laser element of the invention having the above-described construction, the heatsink connected to the substrate side of the semiconductor laser element, and a supply passageway for supplying a cooling medium to the groove, a discharge passageway for discharging the cooling medium from the groove, and means for causing the cooling medium to flow to the groove through both the passageways.